1. Field of the Invention
The present invention relates generally to the separation of designated ions from liquids and has particular use in the desalination of water and in the concentration of dissolved salts in a confined effluent.
2. Description of the Prior Art
The separation of identified ions from a liquid containing those ions has been of interest for many years in connection with various processes. One of these processes is the desalination of water. Another would be the concentration of dissolved salts for use in other economical processes.
Currently, there are five basic techniques that are known and can be used to separate salt and other such dissolved solids from water and these include distillation, reverse osmosis, electrodialysis, ion exchange, and freeze desalination. Distillation and freezing involve removing pure water, in the form of water vapor or ice, from a salty brine. Reverse osmosis and electrodialysis use membranes to separate dissolved salts and minerals from water, while ion exchange involves an exchange of dissolved mineral ions in the water for other, more desirable dissolved ions as the water passes through chemical "resins." A more particular description of each of these techniques is as follows:
Distillation
Salt and mineral-free water can be separated from seawater by vaporizing some of the water from the salt solution and then condensing this water vapor on a cooler surface. This is the same phenomenon that occurs when water vapor or steam inside a warm house condenses on a cold windowpane, or when water vapor condenses to form rain or snow. The vaporization of water molecules can be accelerated by heating the brine to its boiling point and/or reducing the vapor pressure over the brine.
Reverse Osmosis
With reverse osmosis, salty water on one side of a semipermeable membrane is typically subjected to moderate pressures. Pure water will diffuse through the membrane leaving behind a more salty concentrate containing most of the dissolved organic and inorganic contaminates. Reverse osmosis membranes are manufactured commonly in the form of hollow, hair-like fibers or several alternating layers of flat sheet membranes and open spacer fabric which is rolled into a spiral configuration. Membrane selection depends largely on feed water characteristics and membrane costs.
Electrodialysis
With this technique, brackish water is pumped at low pressures between flat, parallel, ion-permeable membranes that are assembled in a stack. Membranes that allow cations to pass through them are alternated with anion-permeable membranes. A direct electrical current is established to cross the stack by electrodes positioned at both ends of the stack. This electric current pulls the ions through the membranes and concentrates them between each alternate pair of membranes. Partially desalted water is left between each adjacent set of membrane pairs. Scaling of the membrane to remove accumulated salt is avoided in most electrodialysis units by operationally reversing the direction of the electrical current around the stacks at predetermined intervals. This reverses the flow of ions through the membranes, so that the spaces collecting salty concentrate begin collecting less salty product water.
Ion Exchange
In this process, undesirable ions in the feedwater are exchanged for desirable ions as the water passes through granular chemicals, called ion exchange resins. Both cation and anion exchange resins are used by industry. For example, cation exchanged resins are typically used in homes and municipal water treatment plants to remove calcium and magnesium ions in "hard" water, and by industries in the production of ultra-pure water. The higher the concentration of dissolved solids in the feedwater, the more often the resins will need to be replaced or regenerated.
Freeze Desalination
When salt water freezes, the ice crystalizes as a pure material leaving the dissolved salt and other minerals in pockets of higher salinity brine. Traditional freezing processes involve five steps: (1) precooling of the feedwater, (2) crystalization of ice into a slush, (3) separation of ice from the brine, (4) washing the ice, and (5) melting the ice. New research efforts are attempting to reduce the number of steps, especially the need to wash the ice crystals.
The afore-noted techniques for separating salt and other dissolved solids from water have met with various degrees of success. With the exception of the ion-exchange technique, each of the other known techniques for desalinating water have the drawback of requiring a considerable amount of energy and thus expense to operate the system. In the case of the ion-exchange technique, chemicals are needed which add to the expense of operating the process.
It is a primary object of the present invention to provide a new and improved method of separating salt or other ions from liquids in a relatively inexpensive manner to provide diluted and concentrated effluents.
It is another object of the present invention to utilize a totally new concept for separating ions from liquids which overcomes the shortcomings of the prior art techniques.